kenmore gas dryer not heating

Model no. 72832100
natural gas dryer.

problem:
not drying clothes enough or not at all, just air drying.

troubleshjooting so far:

took back panel off and vaccumed area.
tested thermal fuse- test ok
took out vent duct and cleaned inside- yes it was full of lint after 10 years.
restarted unit and heater came on the first heat cycle. afterwards, only the igniter, no flame.
took off front panel and looked at photovoltic cell, appears clean, no visual signs of trouble in this area (heated wires, burnt parts etc.)

about 5 years ago I had the thermostat replaced. I believe it could be the same problem. I can get the part locally and install myself, but would like any other testing steps before doing so.

any help would be appreciated.

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kenmore gas dryer not heating

update:

replaced the operating thermostat and have the same results. The dryer works fine when I have it apart. That is it sounds like a switch closes, then a few seconds later the ignitor comes on, then the gas lites. It did this fine for about 20 minutes. Then I put the front panel back on and the gas lit once and that was it.

i also notice a buzzing or humming noise. Possibly the coils for the gas valve are giving out after getting warm???